Take it Gradius(Konami) and others gave you inspiration!. :)
Hey! Lead designer for the game here! I'd call Retroshooti the "anti-Gradius" in that we were purposedly trying to break the rules of that kind of shmup gameplay design; we did get some ideas from Parodius though, and our favorite shmups like Terra Cresta, Star Soldier, Xevious, Recca, etc. Even then, it was always the idea to subvert shmups rather than imitate them!
I'm planning on adding a bit of a homage to the Morning Music on the Bubble System on some obscure place because I find it cute. There's actually a lot of other easter eggs that reference a lot of the game we like on obscure places like some menus. I like Gradius' aesthetics and oldschool Konami in general I find delightfully mystifying but, alas, I'm terrible at most space shooters. Really like Life Force and Guardian Legend on the NES and I really enjoy most of Compile shooters, though! I think I've played through a lot of Gun-Nac on a Famiclone... Anyway, great to hear it works on Linux! We've done all our in-house test on a very low-end machine so we know it at least runs, but it's always great to hear reports from other people. Hope you enjoyed the game!